The present invention concerns oscillator circuits and more particularly coupling arrangements between an oscillator circuit and an amplifier stage following the oscillator circuit.
1. Field of the Invention
Oscillator circuits typically consist of an actual high-frequency generating oscillator and an amplifier stage, the purpose of which is to minimize the loading effect on the actual oscillator. The high-frequency signal generated by the oscillator is not until from the output of the amplifier stage led to the other parts of the system.
2. Description of the Prior Art
Loading of the oscillator effects the noise performance of the oscillator and increases i.a. the phase noise. For this reason, special amplifier stages are used in oscillator circuits between the oscillator and the other parts of the system, so that the loading effect generated by the system and especially the variation of this load would have no influence on the oscillator circuit.
FIG. 1 illustrates a typical oscillator circuit of prior art. The frequency of the voltage-controlled oscillator 2 is adjusted with control voltage V.sub.c that adjusts the junction capacitance of the capacitance diode D1 that belongs to the resonance circuit of the oscillator. The amplifier stage 4 is coupled to the oscillator with coupling capacitor C.sub.k. The output signal of the oscillator circuit is brought to the other parts of the system through the output OUT.
Selection of the coupling capacitor C.sub.k has a significant influence on the characteristics of the oscillator circuit. In principle, the capacitance value of the coupling capacitor should be as low as possible in order to minimize the load on the resonance circuit. The loading effect on the resonance circuit degrades the Q value of the resonance circuit, that causes undesirable effects, such as phase noise. In order to minimize the load, the signal is in most cases taken to the amplifier stage from the oscillator transistor, in oscillator circuits with couplings of the type shown in FIG. 1. A more preferable point for signal output would be the end 6 of the resonance circuit, because at that point the resonance circuit acts like a band pass filter decreasing the noise of the oscillator signal. However, in most cases this can't be done because of the loading effect of the amplifier stage.
Decreasing the capacitance value of the coupling capacitor C.sub.k has another advantage, too: the smaller this capacitance value is, the better the oscillator is isolated from the output OUT of the amplifier stage. Good isolation between the stages is of advantage, because then the variation of the output load of the amplifier stage has no influence on the load the oscillator is subjected to, and so it does not cause any frequency and phase errors.
On the other hand, decreasing of the capacitance value of the coupling capacitor C.sub.k decreases the level of the high-frequency signal that couples to the amplifier stage, whereby it is more difficult to gain the desired output level of the whole oscillator circuit. Thus, the selection of the capacitance value is always a compromise with respect to the oscillator signal interference and the output level. In volume production this is a problem, as the coupling capacitor C.sub.k must always be dimensioned so big that in different devices of the production lot, the output level has always the magnitude of at least a certain, desired value, irrespective of the variations of the values within the tolerances of the components of the circuit. Thus, in most appliances the value of the coupling capacitor C.sub.k is not optimal, whereby the signal quality of the oscillator circuit suffers.
This problem becomes especially emphasized with voltage-controlled oscillators implemented with capacitance diodes. The Q value of capacitance diodes is relatively low, which causes strong noise sidebands compared with e.g. oscillators with fixed frequency implemented with mica capacitors. Oscillators with capacitance diode adjustment are, however, used especially in frequency synthesizers of portable radio devices, because they are suitable to be integrated with usual manufacturing techniques of integrated circuits.